Transceivers are used in many communication systems including wireless devices, for example. A transceiver can be used for both transmit and receive operations in a device. Often, a transceiver is coupled between a baseband processor and an antenna and related circuitry of a system. In the receive direction, incoming radio frequency (RF) signals are received by the transceiver, which downconverts them to a lower frequency for processing by the baseband processor. In the transmit direction, incoming baseband data is provided to the transceiver, which processes the data and upconverts it to a higher frequency, e.g., a RF frequency. The upconverted RF signals are then passed to a power amplifier (PA) for amplification and transmission via an antenna.
Thus the transceiver acts as an interface between baseband and RF domains. Among the various tasks performed by a transceiver are downconversion and upconversion, modulation and demodulation, and other related tasks. Oftentimes, a transceiver is controlled by a baseband processor to which it is coupled. In turn, the transceiver is coupled to a power amplifier, which generates appropriately conditioned RF signals for transmission via an antenna. In the transmit direction, the transceiver provides RF signals to the power amplifier, which amplifies the signals based on gain and ramp information for a given modulation type. Different wireless communication protocols implement different modulation schemes. For example, a Global System for Mobile communication (GSM) system can implement various modulation schemes, including a gaussian minimum shift keying (GMSK) modulation scheme for voice/data. Furthermore, extensions to GSM, such as Enhanced Data rates for GSM Evolution (EDGE) use other modulation schemes, such as an 8 phase shift keying (8-PSK) modulation scheme, in addition to GMSK modulation.
Some PAs in certain wireless protocols operate in a linear mode. Other PAs can operate in a saturated mode. GMSK schemes typically transmit in a saturated mode in which transmitted data is output from the PA with a constant amplitude as measured by power versus time during the useful part of the burst. In contrast, 8-PSK modulation typically requires a linear mode in which a time-varying amplitude is output from the PA as measured by power versus time during the useful part of the burst. However, certain radio architectures including EDGE functionality attempt to use a saturated mode PA for 8-PSK modulation. A technique generally called polar modulation is typically used in these architectures. Polar modulation architectures include separate amplitude and phase/frequency pathways. A variant of polar modulation called polar loop exists in which the separate amplitude and phase/frequency pathways somehow exist as part of a feedback loop. Such polar loop architectures can have stability challenges. In both cases (polar modulation and polar loop), the amplitude pathway contains circuitry whose delay must be matched very closely to the delay through the phase pathway to avoid serious performance degradation. In a production environment, delays should be matched over all variations in process, supply voltage, frequency, output power, and temperature, creating a fundamentally difficult design and manufacturing challenges. As the transfer function of the PA changes over the range of output powers, the polar loop, by virtue of its feedback loop, may become unstable, causing damage to the PA or causing a handset to drop a call. External loop filters complicate calibration by allowing unwanted parasitic coupling to the printed circuit board (PCB) of the handset, PA, and other sources of interference and noise.
The Third Generation Partnership Project (3GPP) specification for compliant second and third generation (3G) mobile stations requires a maximum spectral power density of −54 dBc for 8-PSK at a frequency offset of 400 kHz. Handset manufacturers typically require that no more than −60 dBc be produced from the transceiver. With a delay mismatch of approximately 30 ns the spectrum becomes marginally failing to this specification. Accordingly, challenges exist in implementing 8-PSK features in polar architectures, and a need exists for more suitable radio architectures.